This　study　focused　on　Duihekou　Reservoir,　a　typical　deep-water　reservoir　located　in　the　southeastern　hilly　and　mountainous　areas　of　China.　A　one-year　field　investigation　was　conducted　to　collect　columnar　sediment　column　samples　from　the　main　water　bodies.　Detailed　analyses　were　conducted　on　the　phosphorus　concentrations,　fractions,　interstitial　water　profile　characteristics,　and　spatiotemporal　changes　of　fluxes　across　the　sediment-water　interface.　The　results　showed　that　the　average　total　phosphorus　(TP)　concentrations　in　the　surficial　sediment　of　the　reservoir　ranged　from　470.8　to　1012.3mg/kg,　with　an　average　of　688.4　±　186.48mg/kg.　An　increasing　trend　of　TP　concentrations　was　discovered　from　the　upstream　to　downstream.　The　most　severe　phosphorus　pollution　was　discovered　around　the　dam.　The　phosphorus　fractions　followed　a　descending　order　of　Al-P　(352.61mg/kg)　＞　Fe-P　(98.10mg/kg)　＞　Res-P　(88.77mg/kg)　＞　Ca-P　(72.42mg/kg)　＞　Org-P　(33.38mg/kg)　＞　Ex-P　(1.64mg/kg).　Overall,　the　sediment　in　the　entire　reservoir　acted　as　a　source　of　phosphorus　to　the　water,　with　the　highest　risk　of　release　in　autumn　and　most　of　the　sediment　in　winter　presenting　as　a　sink　of　phosphorus.　The　average　static　release　flux　and　diffusion　flux　of　phosphorus　were　(0.81±2.34)　and　(2.15±3.47)　mg/(m2·d),　respectively.　The　annual　phosphorus　release　amounts　obtained　by　the　two　methods　were　1.92　and　3.67　tons　respectively,　with　the　highest　release　amount　in　autumn.　The　conversion　of　Fe-P　might　be　an　important　factor　controlling　phosphorus　release　from　the　sediment.　The　anaerobic　dissolution　and　release　of　Fe-P　caused　by　oxygen　stratification　will　increase　the　phosphorus　concentration　in　the　lower　layer　of　the　water　column,　which　would　subsequently　increase　the　phosphorus　concentration　in　the　whole　water　column　after　thermal　stratification.　After　a　large　amount　of　phosphorus　is　released　in　autumn,　more　attention　should　be　paid　to　the　phosphorus　concentration　in　the　water　influenced　by　the　subsequent　thermal　inversion.　©　2024　Chinese　Society　for　Environmental　Sciences.　All　rights　reserved.